FIG. 5 is a cross-sectional view illustrating the structure of a conventional buried ridge type semiconductor laser including an n type GaAs substrate 1, an n type Al.sub.0.48 Ga.sub.0.52 As lower cladding layer 2, an undoped Al.sub.0.13 Ga.sub.0.87 As active layer 3, a first p type GaAs contact layer 15a, a p type Al.sub.0.48 Ga.sub.0.52 As upper cladding layer 4, a ridge stripe structure 9 comprising the first p type GaAs contact layer 15a and the upper part of the upper cladding layer 4, an n type GaAs current blocking layer 7 burying the p type Al.sub.0.48 Ga.sub.0.52 As upper cladding layer 4 of the ridge stripe structure 9, a second p type GaAs contact layer 15b burying the p type GaAs contact layer 15a of the ridge stripe structure 9, an n side electrode 16, and a p side electrode 17.
FIGS. 6(a) to 6(d) are cross-sectional views illustrating fabrication of a conventional buried ridge type semiconductor laser in which the same reference numerals used in FIG. 5 designate the same or corresponding elements. Reference numeral 8 designates an insulating film of stripe configuration extending in the &lt;110&gt; direction and comprising a material such as SiON or SiN, and reference numeral 10 designates a dislocation.
Next, the fabrication method will be described. First, the n type Al.sub.0.48 Ga.sub.0.52 As lower cladding layer 2, the undoped Al.sub.0.13 Ga.sub.0.87 As active layer 3, the p type Al.sub.0.48 Ga.sub.0.52 As upper cladding layer 4, and the first p type GaAs contact layer 15a are successively epitaxially grown in a first epitaxial growth step on the (001) surface of the n type GaAs substrate 1, whereby the semiconductor layer structure shown in FIG. 6(a) is produced. This first epitaxial growth step employs ordinary methods used in fabricating semiconductor devices, such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Next, the SiON film 8 having a stripe configuration extending in the &lt;110&gt; direction is formed on the semiconductor layer structure produced by the first epitaxial growth step (FIG. 6(b)). This SiON film 8 should have a thickness of 50 to 100 nm and is commonly produced by plasma CVD. Next, using this insulating film 8 as an etching mask, the semiconductor layer structure is etched into the p type Al.sub.0.48 Ga.sub.0.52 As cladding layer 4 to produce the ridge stripe structure 9 (FIG. 6(c)). Then, using the insulating film 8, the n type GaAs current blocking layer 7 is selectively grown, in a second epitaxial growth step employing MOCVD, to bury the p type Al.sub.0.48 Ga.sub.0.52 As cladding layer 4 of the ridge stripe structure 9, at a temperature of about 650.degree. C. The second p type GaAs contact layer 15b is then grown to bury the first p type GaAs contact layer 15a of the ridge stripe structure 9. Then, the semiconductor structure is cooled to about 500.degree. C. (FIG. 6(d)) and, finally the p side electrode 17 and the n side electrode 16 are formed on the surfaces of the contact layers 15a and 15b and on the rear surface of the substrate 1, respectively, whereby the semiconductor laser shown in FIG. 5 is completed.
Next, operation of the semiconductor laser will be described with reference to FIG. 5. When a bias voltage is applied to the p side electrode 17 and the n side electrode 16 so that the first p type GaAs contact layer 15a and the second p type GaAs contact layer 15b become positive, since in a region where the ridge stripe structure 9 does not exist there is a thyristor structure (p-n-p-n junction) comprising the second p type contact layer 15b, the n type current blocking layer 7, the p type upper cladding layer 4, and the n type lower cladding layer 2, an electrical current only flows through the ridge stripe structure 9 and not through the thyristor structure. Electrons and holes injected into the region of the undoped Al.sub.0.13 Ga.sub.0.87 As active layer 3 under the ridge stripe structure 9 produce radiative recombination, i.e., photons. If the carrier injection level is raised, a stimulated emission begins and produces a laser oscillation.
In the prior art fabricating process as described above, it is likely that dislocations may occur where an edge of the insulating film 8 along the stripe length direction and the second p type contact layer 15b come in contact as shown in FIG. 6(d). This may be due to structural forces, such as a shearing stress, between the insulating film 8 and the first p type GaAs contact layer 15a, and may happen during the second epitaxial growth step for burying the ridge stripe 9 or during the cooling process after the second epitaxial growth step. These dislocations may grow parallel to the (111) plane and may penetrate entirely through the region of the active layer 3 directly beneath the ridge stripe structure. If a voltage is applied to such a device having a penetrating dislocation 10, the dislocation will propagate along the (011) plane and the &lt;100&gt; dark line will grow and degrade the laser characteristics rapidly. Therefore, it will be extremely difficult to obtain a semiconductor laser having a long lifetime.
A structure of a semiconductor laser devised to solve this problem is disclosed in Japanese Published Patent Application Hei. 3-225985. FIG. 7 is a cross-sectional view illustrating the structure of this semiconductor laser. The semiconductor laser of FIG. 7 includes an n type GaAs substrate 21, an n type In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P lower cladding layer 22, an In.sub.0.5 Ga.sub.0.5 P active layer 23, a first p type In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P upper cladding layer 24a, a second p type In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P upper cladding layer 24b, a p type GaAs contact layer 25, an etch stopping layer 26 comprising a superlattice layer including alternating layers of p type In.sub.0.5 Ga.sub.0.5 P and p type In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P, and an n type GaAs current blocking layer 27.
The method of fabricating this semiconductor laser will be described as follows. The n type lower cladding layer 22, the active layer 23, the first p type upper cladding layer 24a, the etch stopping layer 26, and the second p type upper cladding layer 24b are successively grown on the n type GaAs substrate 21 in a first epitaxial growth step. Next, using an insulating film having a stripe shape (not shown in the figure) as a mask, the second p type upper cladding layer 24b is etched to reach the etch stopping layer 26 and form a ridge stripe shape. Then, the current blocking layer 27 is grown in a second epitaxial growth step to bury the ridge stripe structure and, after removing the stripe-shaped insulating film, the p type contact layer 25 is formed on the second p type upper cladding layer 24b and the current blocking layer 27. For this semiconductor laser, during the second epitaxial growth step for forming the current blocking layer 27 or during the cooling process following the second epitaxial growth step, through dislocations originating at an edge of the stripe shaped insulating film in the stripe length direction and growing along the (111) plane reach the etch stopping layer 26. However, since the etch stopping layer 26 is a superlattice layer, although some of the dislocations pass through some of the layers of the interfaces between the p type In.sub.0.5 Ga.sub.0.5 P layer and the p type In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P layer, the dislocations are ultimately redirected by the intermediate interfaces to a direction parallel with the surface of the etch stopping layer 26, i.e., the (001) surface. Thus, they do not penetrate the active layer 3 below the second p type upper cladding layer 24b having a ridge stripe configuration. This eliminates the problem of generation of the &lt;100&gt; dark lines that deteriorate laser characteristics.
As described above, in the prior art semiconductor laser shown in FIG. 7, the etch stopping layer 26 comprising a superlattice is inserted between the first p type upper cladding layer 24a on the active layer 23 and the ridge stripe-shaped second p type upper cladding layer 24b, so that growth of dislocations penetrating into the active region is prevented. However, in the usual buried ridge type semiconductor laser, since the thickness of the upper cladding layer between the ridge stripe structure and the active layer is quite thin, about 0.3 .mu.m, if the etch stopping layer 26 is placed under the second p type upper cladding layer 24b in the ridge stripe structure, the laser light generated at the active region of the active layer 23 and usually broadened to about 0.5 .mu.m from this active region will be absorbed by the etch stopping layer 26 comprising the superlattice. This results in a large optical loss in the waveguide, seriously adversely affecting the characteristics of the semiconductor laser.